It has been known for a long time that the output frequency of a crystal oscillator will vary from its expected, nominal, frequency as the temperature of the oscillator varies. The temperature ranges of such oscillators might be from minus thirty degrees centigrade to plus seventy degrees. Uncompensated frequency variations might be as much as 5-20 parts per million over the above temperature range.
In the prior art, with respect to AT cut crystals, passive compensation networks have been used. These networks include capacitance added in series with the crystal to reduce the variation in output frequency. While added series capacitance is effective in reducing the frequency variation, the problem of determining the correct value or values to add has been and continues to be without a satisfactory solution.
Values of temperature compensation components have often been arrived at by a trial and error iterative process. A "best guess" is made at the compensation values. The circuit is built and then tested over the temperature range. This testing requires both the use of cooling and heating equipment as well as the use of measuring equipment to determine the output frequency of the oscillator at various temperatures.
Once the actual frequency variation has been determined, the compensation elements can be changed, if necessary, to bring it into specification. The test must the be rerun.
The above iterative process can take a relatively long time and can require six or seven iterations. The process of changing components, including soldering and unsoldering can have a deleterious effect on the long term reliability of the circuit. In addition, the circuit must be physically arranged to enable this component replacement to take place. This of course imposes limitations on the physical size of the circuit.
As an alternate, an integrated circuit has been developed to compensate AT cut crystal oscillators. This circuit is disclosed in U.S. Pat. No. 4,254,382 assigned to the assignee of the present invention. The integrated circuit provides a temperature varying voltage which is applied to a varactor diode. The integrated circuit will provide very precise compensation over a wide range of operating temperatures.
As is illustrated in FIG. 1 of the abovenoted patent, for temperature ranges between minus ten degrees centigrade and plus fifty degrees centigrade the variation in frequency with temperature is essentially a linear function with a negative slope. Many electronic circuits have operating ranges that correspond to this temperature range. For such circuits, passive compensation networks are often preferred because of cost, space or power considerations. There thus continues to be a need for a better method to determine values of compensation capacitors in passive compensation networks.
It is therefore an object of the present invention to provide a method of adjusting the values of passive temperature compensation components which does not require removing and replacing such components from the circuit.
It is a further object of the present invention to provide a method of adjusting the values of passive temperature compensation components which determines in an interactive fashion acceptable component values based on a circuit model rather than by a trial and error process.
It is a further object of the present invention to determine oscillation frequency values to which a temperature compensation capacitor and a trim capacitor can be adjusted by using a laser beam and without removing either capacitor from the oscillator circuit.